A control apparatus for an alternating-current synchronous motor generally requires a detector which detects an angle of rotation of a rotator to perform drive control of a synchronous electric motor. However, a drive apparatus using a detector has problems as described as examples below.
Firstly, presence of the detector causes increase in volume of an entire drive system. This increase hinders enhancement of output from a synchronous machine in a limited installation space. Secondly, maintenance work for the detector itself is required. Maintenance work efficiency therefore deteriorates. Thirdly, noise is superimposed on a signal line from the detector. Therefore, detected values are disturbed and control performance deteriorates. Fourthly, most of detectors require power supplies for driving themselves, and a power supply of a system different from that for driving the synchronous machine needs to be provided. This is a factor which increases burdens on a power-supply installation space, a power supply line, and costs.
From reasons as described, a control method has been developed in which an angle of rotation is estimated by calculation based on electric information such as a current or a voltage to drive the synchronous machine, and drive control is performed by using the estimated angle of rotation without using a detector. This control method is generally referred to as “sensorless control”.
As prior art which proposes a method especially effective for a standstill state and a low-speed state in a control apparatus for a synchronous machine which comprises such a sensorless control unit, there is a system which drives a synchronous machine by a PWM inverter. In the system, a high-frequency voltage command at a sufficiently high frequency compared with an operating frequency of the synchronous machine is superimposed on a control command of a control apparatus which controls an inverter. Such an apparatus obtains estimated error information for an angle of rotation by detecting a component corresponding to a superimposed high-frequency voltage command, from a high-frequency current response generated because of the high-frequency voltage command. By using the information, the angle of rotation is estimated.
The control apparatus as described above for a synchronous machine can control the synchronous machine without using a sensor and is effective in that maintenance property improves at low costs. However, as described above, in the method of detecting a component corresponding to a high-frequency voltage command of a high-frequency current response, a desired high-frequency current needs to be flowed, and has a problem of extreme increase in loss and noise in comparison with a system using a sensor. In addition, to stably estimate a rotational phase angle, the amplitude and frequency of a high-frequency command and a high-frequency superimposition method need to be finely adjusted. In order to perform stable operation by combining a motor and a control apparatus in actual, complex time-consuming adjustment is required.
Further, in a method which has been proposed to solve the problems of the prior art, a high-frequency component of a current generated by a voltage output from a PWM inverter is calculated, and an angle of rotation is estimated by using a spatial distribution of inductances.
In the proposed method as above, when a rotational phase angle is estimated by using the spatial distribution of inductances, estimation is performed with use of a high-frequency current value which is generated by switching in actual. Therefore, a high-frequency current caused by PWM switching needs to be observed. However, the current needs to be sampled at timings close to switching in order to observe a high-frequency current caused by switching. Further, in the foregoing method, there is difficulty in observing the current with high accuracy under influence of spike noise caused by switching, and particular consideration is required for current sampling management and noise solution.
The embodiments described herein are to solve the problems described above, and provide a sensorless control apparatus which suppresses loss and noise caused by a high-frequency current to detect rotation of a rotator, without requiring a particular care for observation of a current.